There might be a problem reading those files. When you click Import and the import window opens, please select the ogg file and tell us what happens when you click "play" on the right side under the small preview.
Can you play those files with GStreamer from the command line? Try: gst-play-1.0 PATH-TO-OGG-FILE Please see http://developer.pitivi.org/Bug_reporting.html?stack-traces-for-crashes-and-deadlocks for how to provide a stack trace when Pitivi hangs.
